The present invention relates to an automatic frequency control apparatus in a digital modulation communication system.
U.S. Pat. No. 5,020,131 "TRANSPONDER FREQUENCY OFFSET COMPENSATION WITH RECURRENTLY ASSIGNED UNIQUE WORDS" discloses a conventional technique for an automatic frequency control apparatus in a communication system in which a unique word is located at a proper position of transmission data, and a carrier is digitally modulated on the basis of the transmission data to transmit the carrier. This conventional automatic frequency control apparatus will be described below with reference to the block diagram shown in FIG. 3.
Referring to FIG. 3, a conventional automatic gain control apparatus is constituted by a frequency converter 1 for converting an input reception RF (Radio Frequency) signal into an IF (Intermediate Frequency) signal, a demodulator circuit 8 for demodulating the IF signal to output demodulated data, unique word detectors 9 to 12 for detecting unique words, a reception frequency control circuit 13 for controlling the oscillation frequency of a frequency synthesizer 15 on the basis of reception channel setting information and outputs from the unique word detectors 9 to 12, a reception channel setting circuit 14 for outputting the reception channel setting information set in the reception frequency control circuit 13, and a frequency synthesizer 15 for outputting a local oscillation frequency to the frequency converter 1.
The input RF signal is mixed with a local oscillation frequency output from the frequency synthesizer 15 to obtain an IF signal, and the IF signal is input to the demodulator circuit 8. The demodulator circuit 8 tunes its frequency to the frequency of the carrier component of an output from the frequency converter 1, extracts a clock from the input IF signal, demodulates data, and outputs demodulated data.
The unique word detectors 9 to 12 are used to detect unique words included in the demodulated data. As the unique word detectors 9 to 12, an arbitrary number of unique word detectors are prepared for different reception channels or different reception frequencies. The unique word detectors detect different unique words, respectively. The reception frequency control circuit 13 detects the channel or frequency of an input signal depending on a specific one of the arbitrary number of unique word detectors which detects a unique word, and controls the frequency set by the reception channel setting circuit 14 to output the frequency to the frequency synthesizer 15. The frequency synthesizer 15 changes an oscillation frequency in accordance with a value indicated by the frequency control circuit 13. In this manner, the automatic frequency control apparatus is realized.
In the conventional automatic frequency control apparatus, an arbitrary number of unique word detectors must be prepared for different reception channels, i.e., different reception frequencies, respectively. For this reason, the frequency accuracy of automatic frequency control is determined by the number of unique word detectors, and the number of unique word detectors is increased to improve the frequency accuracy. In addition, since unique words are detected after a demodulation process, a circuit arrangement in a communication system for digitally demodulating a plurality of subcarriers with transmission data and synthesizing them to transmit the obtained carrier is complicated, thereby avoiding a reduction in circuit scale and circuit integration.